In general, as methods for preparing a polycarbonate (hereinafter referred to as "PC"), there are known a method which comprises directly reacting an aromatic dihydroxy compound such as bisphenol A with phosgene (an interfacial method), and another method which comprises carrying out an ester exchange reaction between an aromatic dihydroxy compound such as bisphenol A and a carbonic acid diester such as diphenyl carbonate in a melting state (a melting method).
In the PC preparation methods, the above-mentioned interfacial method has some problems. For example, (1) harmful phosgene must be used, (2) chlorine-containing compounds such as hydrogen chloride and sodium chloride as by-products accelerate the corrosion of the manufacturing apparatus, and (3) it is difficult to separate impurities such as sodium hydroxide with which the resin is contaminated and which has a harmful influence on the physical properties of the polymer.
On the other hand, the above-mentioned melting method has an advantage that the PC can be manufactured more inexpensively as compared with the interfacial method. In the melting method, however, the reaction is done at a high temperature of 280.degree. to 310.degree. C. for a long period of time, and therefore this method has a large drawback that a problem regarding the coloring of the resin is unavoidable.
In such a melting method, various improved techniques have been suggested in order to decrease this coloring phenomenon. For example, Japanese Patent Publication No. 39972/1986 and Japanese Patent Application Laid-open No. 223036/1988 have disclosed methods using specific catalysts. Furthermore, Japanese Patent Application Laid-open Nos. 151236/1986 and 158719/1987 have disclosed methods in which an antioxidant is added in the second half of reaction. Moreover, in Japanese Patent Application Laid-open No. 62522/1986, the use of a twin-screw bent type kneading extruder in the second half of reaction has been disclosed, and in Japanese Patent Application Laid-open No. 153925/1990, an improved technique regarding a process has been disclosed in which a transverse type stirring polymerizer is used. Additionally, in Japanese Patent Application Laid-open No. 175722/1990, a method has been disclosed in which the content of hydrolyzable chlorine in a monomer is controlled to a certain level or less.
However, the problem of the coloring has not completely been solved yet, and a technique for obtaining the satisfactory PC has not been attained so far.